Cyanoacrylates (CAs) is the generic name for a family of resistant fast acting adhesives based on esters of 2-cyanoacrylic acid. The structure of the monomer is as follows:
wherein R is usually an alkyl group such as, for example, methyl, ethyl, butyl, or octyl, or an alkoxyalkyl group, for example, 2-methoxyethyl or 2-ethoxyethyl.
Such compounds have been well known for some time, as described in, for example, S. Ebnesajjad Ed., Adhesives Technology Handbook, William Andrew, Norwich, 2008.
CAs are the reactive monomers and the main components of so-called ‘super glues’ or ‘instant adhesives’. They are unique with regard to speed of cure and ability to bond almost every substrate most generally without the need for an additional energy stimulus such as heat or light. This type of adhesive is widely used by consumers, professional craft workers and in industrial assembly applications.
Methylidene malonates are another class of related instantly polymerisable monomer, they are acrylates but with a second ester group on the alpha carbon atom as disclosed in, for example, KR-A-2012/0131802 or KR-A-2012/0128004.
In certain applications instantly polymerizing monomers exhibit some disadvantages that detract from the even greater applicability for these materials. One disadvantage exhibited by CAs relates to the presence of uncured excess adhesive outside a bonded part. Uncured material will stick to anything else that comes in contact with it—for example finished or semi-finished goods, machine parts, operator's hands etc. This problem has been addressed by dual curing CAs and partly resolved with the advent of light- or photo-curing CAs. Whereas CAs generally need no assistance when curing tightly fitting parts, they benefit from means to enable cure in bulk, or outside tightly fitting parts, for example to seal joints and prevent inadvertent adhesion to anything that may contact the parts to which glue is applied. A typical example of where a secondary curing mechanism is invoked is in the assembly of medical devices, for example, when hypodermic needles are mounted into the hubs of syringes in a fast running production process. In the latter case, the CA must still function normally (that is, cure as an instant adhesive without light), but additional ultraviolet (UV) curing enables full and fast cure of any excess adhesive and effectively seals and strengthens the joined parts.
Certain CA compositions with an additional light cure capability have been described in prior art and a few specific types are articles of commerce, but several problems remain unsolved, such as the provision of a rapidly curing composition mediated indirectly through a photoinitiated radical mechanism, as mentioned in International patent application WO-A-03/064483.
A first problem relating to current light curing CAs results from the mechanism of cure. CAs polymerize easily by nucleophilic, anionic or zwitterionic mechanisms, and do not respond in a practical manner to free radical homo- or copolymerization unlike common acrylates, like methyl methacrylate or butyl acrylate, among others. Thus it is necessary first to suppress the more dominant anionic cure to invoke true radical homopolymerisation of CAs, by addition of acidic materials as noted by Bevington et al., European Polymer Journal, 1976, 12, 255-267, for methyl cyanoacrylate, after which homopolymerisation could be induced thermally in the presence of radical initiators at 60° C.
In Cloete et al., International Journal of Adhesion and Adhesives, 2010, 30(4), 208-213, the use of dibenzoylferrocene as an anionic photoinitiator of ethyl 2-cyanoacrylate (ECA) is disclosed. In Yamaguchi et al., Macromolecules, 2000, 33, 1152-1156, mono- and dibenzoylferrocenes have been demonstrated to be photoinitiators for CA when used alone. However these compounds are highly coloured and do not exhibit useful long-term stability and are not suited to commercial formulations. In Yamaguchi et al., Coordination Chem. Rev, 2007, 251, 515-524 the use of ruthenocene is disclosed as well as its benzoyl derivatives as sole initiators for CA photopolymerisation that are faster than equivalent sole iron based equivalents. However these suffer the some of the same difficulties as their ferrocene analogues when used as sole components in a photoinitiator system with regard to lack of long term stability and furthermore they are highly expensive.
In International patent application WO-A-98/38260 a radiation-curable composition comprising a CA component, a metallocene component and radical photoinitiator components such as acylphosphine oxides, among others, is disclosed. According to A. Ciechacka, “Analysis and Characterisation of an Acylphosphine Oxide Photoinitiator”, PhD thesis, Dublin City University, 2011, the radical photoinitiator alone does not invoke effective CA polymerization directly, but rather serves to facilitate the subsequent creation of the eventual initiating species suited to CA polymerization.
A serious disadvantage of the requisite acylphosphine oxide radical initiators is their susceptibility to hydrolysis in acidic conditions as highlighted in the abovementioned thesis study and also indicated in related systems (dental materials) in European patent application EP-A-1749513. This process results in premature decomposition of this co-initiator component even in the dark, leading to formulations that lose photosensitivity with storage time, and formulations that require refrigerated storage to achieve acceptable shelf life—in sum, unstable or unworkable CA formulations. It is to be noted that reactive CA formulations must be stabilised by small amounts of strong acids, so formulation components in CAs must be acid stable.
A second problem relating to light curing CAs results from the nature of the CA monomer per se. The most widely used monomer in all categories of instant adhesives is ethyl CA (ECA). In the specialized category of light curing CAs, only ECA versions are commercially available (such as Loctite® 4305, 4310, 4311, Threebond® 1771E, 1773E, TB1741), even though reference is made to other CA types as disclosed, for example, in European patent application EP-A-0769721, yet no examples have ever been provided. In very many situations ECA based adhesives are fit for purpose, however in certain circumstances other types may be preferred. ECA possess a relatively high vapour pressure, is a strong lachrymator and must be labelled as an irritant. High vapour pressure gives rise to a white staining (so-called ‘blooming’) when polymerized vapours condense on parts generating unsightly marks on manufactured goods. Lachrymatory characteristics make it difficult to work with ECA on a continuous basis except when used in automated and properly ventilated systems. Both lachrymatory and irritant properties mean that ECA based formulations are less well suited to large area coating applications where large open surfaces are exposed or to application in domestic situations. All of these issues pertain not only to general ECA based instant adhesives, but also to light curing versions based on this monomer, so that even if normal applications of ECA do not include application to large open surfaces, such monomers would still present problems if light curing coatings were to be derived from them. Odourless, non-staining, non-irritant instant adhesives are well known, as disclosed for example in U.S. Pat. No. 4,321,180, based on monomers such as the alkoxyalkyl CAs, for example, methoxyethyl CA (MECA). However light curing versions of MECA are not commercially available. Even though reference has been made to such materials in the prior art of light curing ECA formulations, the reality is that alkoxyalkyl CAs are inherently sensitive to autoxidation which raises additional problems as disclosed in U.S. Pat. No. 4,321,180. Stable alkoxyalkyl CA formulations thus require special attention to radical stabilization, which further complicates formulating light-curing versions, particularly those containing mixtures with either radical co-initiators, or with radically polymerisable acrylic co-monomers.
For ECA formulations, even if suboptimal with regard to every desirable product attribute, it still would be highly desirable to further optimize light sensitive versions using this monomer, for example, for bulk cure. However, to have access to stable, odourless, non-staining, non-irritant, fast light curing alkoxyalkyl CAs, especially with good sensitivity to visible light would be still more desirable. The latter would be uniquely practical for coating applications and have the significant advantage over:                a) conventional light curing acrylic materials, since CA polymerization is not inhibited by ambient air and hence they cure ‘dry-to-touch’, or ‘tack free’ under normal conditions even in ultrathin coatings and,        b) current or improved light curing ECA formulations due to their non-irritant, non-lachrymatory nature.        
The combination of visible light sensitivity, lack of odour and non-irritant character would make MECA based formulations particularly important for large surface coating, or coating in normal ambient conditions for example, in cosmetic applications for nail lacquers for home or salon use, or for use as bulk resins in 3 dimensional printing (3DP) or so-called or so-called Continuous Liquid Interface Printing (CLIP), as disclosed in International patent application WO-A-2014/126834, especially for office based use.
A further potential problem relating to light curing CAs relates to the performance of the final cured materials. Currently commercially available photocurable ECA formulations do not crosslink upon cure. Non-crosslinked cured adhesives have limited durability and in some applications this feature is required, for example if bonded parts have to be sterilized under arduous conditions then they must be robust, or if bulk cured material require high mechanical integrity, then robustness is advantageous. Crosslinking may be required in resin and sealant applications and even to a degree in temporary coatings, such as cosmetic lacquers or protective coatings where durability in the form of scratch resistance is desirable at least for a while. For more robust resins, adhesives, sealants and coatings, photocrosslinking of part or all of the formulation by homo- or co-polymerization would be highly advantageous irrespective of CA type.
There is, thus, a particular need in the CA formulation art, for a light sensitive initiating system that is stable in acidic conditions over prolonged times and is preferably sensitive to both ultraviolet light (UV) and visible light (VL), and which is suitable for preparing multipurpose cured bulk material or films.